Uncross-linked elastomers

Figure 17 Calculated stress-relaxation behavior at 298 K for five uncross-linked elastomers of M = 200,000 EP, ethylene-propylene (56 44) styrene-butadiene (23.5 76.5), SB natural rubber, N butyl and dimethyl siloxane.

FIGURE 4.9 Dynamic storage modulus and tan 8 as a function of angular frequency for a typical uncross-linked elastomer. 

Since the excellent work of Moore and Watson (6, who cross-linked natural rubber with t-butylperoxide, most workers have assumed that physical cross-links contribute to the equilibrium elastic properties of cross-linked elastomers. This idea seems to be fully confirmed in work by Graessley and co-workers who used the Langley method on radiation cross-linked polybutadiene (.7) and ethylene-propylene copolymer (8) to study trapped entanglements. Two-network results on 1,2-polybutadiene (9.10) also indicate that the equilibrium elastic contribution from chain entangling at high degrees of cross-linking is quantitatively equal to the pseudoequilibrium rubber plateau modulus (1 1.) of the uncross-linked polymer. 

The large scale molecular motions which take place in the rubber plateau and terminal zones of an uncross-linked linear polymer give rise to stress relaxation and thereby energy dissipation. For narrow molecular weight distribution elastomers non-catastrophic rupture of the material is caused by the disentanglement processes which occur in the terminal zone, e.g., by the reptation process. In practical terms it means that the green strength of the elastomer is poor. 

Plasticized polyvinyl chloride can be regarded as the first thermoplastic elastomer, as it is used in an uncross-linked form. Because of the lack of cross-linking, this material exhibits high rates of creep and stress relaxation. As with other thermoplastic elastomers, these disadvantages worsen as the temperature is... 

Table 1 summarizes important influential factors, listed separately for uncross-linked plastics (thermoplastics) and cross-linked plastics (elastomers/duroplastics). 

The dispersed elastomer particles are uncross-linked (TPE-O or TPO), but may also be cross-linked (TPE-V). The cross-linking (vulcanization) occurs during compounding in closed mixers (batchwise) or with twin-screw extruders (continuous). 

Uncross-linked plastics (thermoplastics, thermoplastic elastomers) may either swell or dissolve in a given medium. 

The two moduli are functions of temperature and frequency. The parameters G and tan 8, for a typical uncross-linked (unvulcanized) rubber or elastomer, are shown in Fig. 4.8 as a function of temperature (with frequency on the order of 1 Hz). The parameters are plotted against frequency, at a constant temperature, in Fig. 4.9. Thus, one observes that an increase in frequency (equivalent to an increase in strain rate) is similar to a decreased temperature. 

The high hydrophobicity of soluble fluorinated polyphosphazenes has drawn attention to their possible use as surface coatings in applications where insoluble fluoropoly-mers like Teflon cannot be used. This also applies to PNF-type elastomers that, in the uncross-linked state, are highly adhesive. One application that has shown promise is as ice-phobic coatings on helicopter rotor blades. Another reported application is the use of trifluoroethoxy-substituted polyphosphazenes for coating biomedical devices. 

Viscoelasticity is perhaps the most ubiquitous characteristic of high molecular weight polymers at temperatures above Tg. Here we consider the implications of coupling rubber elasticity and mesomorphism via synthesizing covalent networks from conventional elastomers (siloxanes, isoprenes, etc.) and typical MLC mesogenic cores such networks are thermotropic PLCs. At low levels of crosslink densities in the PLC network, there is no appreciable change in the transition temperatures (Tg, Td, etc.) from those of the uncross-linked PLC (and its ancestral MLC) [68]. Below Td, rather modest mechanical deformations (extension ratio A < 1.5) may convert an initial random and disclination-ridden texture into a... 

High molecular weight polyisobutylene has fair tensile strength but suffers from the disadvantage of cOTisiderable cold flow. A copolymer of isobutylene with some isoprene for cross-linking is, therefore, used as a commercial elastomer and called butyl rubber. The isoprene is present in the copolymer in only minor proportions (1.4—4.5%). The uncross-Unked material is very similar to polyisobutylene. Copolymers of isobutylene with other dienes are also called butyl rubbers. They can also be terpolymers, where the third component may be cyclopentadiene for improved ozone resistance. 

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